1. Field of the Invention
The present invention is directed to a phantom for measuring slice thicknesses, slice sensitivity profiles and the axial modulation transfer function (MTF) in an X-ray computed tomography apparatus.
2. Description of the Prior Art
In X-ray computer tomography (CT), the measurement field is limited in the axial direction by intense gating of the useful x-ray beam and a slice having an allocated thickness, referred to as the slice thickness, is thus defined. For numerous reasons, this measurement field gating is experimentally identified or checked with suitable measurement means. The measurement means employed which have been conventionally employed allow the measurement of the slice sensitivity profile in a spiral CT exposure and an exact acquisition of extremely thin slices (1 mm and less) in a single-slice exposure inadequately or not at all.
In conformity with standards, the slice profiles in CT are measured with phantoms known as ramp phantoms (for example, DIN 6868, Part 53). These are thin, highly attenuating objects lying obliquely in the examination slice, usually metal wires or strips. This is well-suited for single-slice exposures having standard slice thickness of 2 mm and more. For thinner slices, relatively high systematic errors occur due to the finite thickness of the metals. These test bodies are not suited at all for volume exposure with spiral CT since they produce pronounced image errors (see Ch. Sxc3xcxcex2, W. A. Kalender, xe2x80x9cPerformance Evaluation and Quality Control in Spiral CTxe2x80x9d, in Medical CT and Ultrasound: Current Technology and Applications, pp. 467-485, American Association of Physicists in Medicine/Advanced Medical Publishing, 1995).
Attempts have been made for several years to find a suitable measurement phantom. In 1994, thus, W. A. Kalender et al., xe2x80x9cA Comparison of Conventional and Spiral CT: An Experimental Study on the Detection of Spherical Lesionsxe2x80x9d, in Journal of Computer Assisted Tomography, 18(2): 167-176, March/April, 1994, Ravel Press Ltd., New York, describes the measurement of the axial image [sic] function (MTF), i.e. of the slice profile, with minute lead balls.
Small copper or steel balls were employed in 1995 by Davros et al., xe2x80x9cDetermination of Spiral CT Slice Sensitivity Profiles Using a Point Response Phantomxe2x80x9d, in Journal of Computer Assisted Tomography, 19(5):838-843, September/October, 1995, Lippincoft-Raven Publishers, Philadelphia. Such a phantom has been offered in the meantime by xe2x80x98Nuclear Associates, USAxe2x80x99.
A similar phantom was disclosed in 1992 in U.S. Pat. No. 5,164,978 for measuring the MTF in the image plane; however, it could also be employed for measuring the axial MTF.
Fundamentally, these ball phantoms are very well-suited for the measurement but have the disadvantage that the subject contrast is extremely low: the expanse dare not amount to more than {fraction (1/10)} of the slice thickness, so that the contrast already drops to below 10% due to the partial volume effect. As a result of the MTF in the image plane, the contrast drops again to about 1%, i.e. drops overall to approximately 0.1% of the original value and thus deteriorates the measurement precision of the method. Different ball diameters are therefore employed dependent on the slice thickness. Moreover, the manufacture of minutes balls (100 through 500 xcexcm) is not simple to realize technologically. For example, the test phantom of xe2x80x98Nuclear Associatesxe2x80x99 contains a piece of copper wire that inadequately simulates the spherical shape and therefore produces pronounced measurement errors.
DIN 6866, Part 10, proposes a phantom based on a plastic body with a shallow cavity. The contrast between air and plastic, however, is relatively low. The cavity must therefore have a large diameter, which in turns makes the exact positioning parallel to the image plane more difficult. Similar problems occur in the phantom of the type initially described, as reported by W. A. Kalender et al. Polacin [sic], xe2x80x9cPhysical performance characteristics of spiral CT scanningxe2x80x9d, in Med. Phys. 18(5):910-915, September/Octobetr 1991, am. Assoc. Phys. Med., 1995, whereby an aluminum foil having a diameter of 16 cm is placed between two plastic disks having the same diameter. Difficulties in the alignment can also occur given a phantom of the type initially described that is known from U.S. Pat. No. 5,793,835.
In conjunction with the measurement of slice sensitivity profiles and axial modulation transfer functions, the aforementioned U.S. Pat. No. 5,164,978 as well as U.S. Pat. No. 4,972,451 disclose that the expanse of the structures utilized in phantoms for this calibration should be smaller than the smallest or thinnest structure to be measured. U.S. Pat. No. 5,164,978 also discloses the employment in this context of material that highly attenuates X-radiation. This employment is also known from U.S. Pat. No. 5,561,698.
An object of the present invention is to provide a phantom of the type initially described which provides adequate subject contrast but wherein the dimension in the slice plane is nonetheless slight.
This object is inventively achieved by a phantom for measuring slice thicknesses, slice sensitivity profiles and the axial modulation transfer function in an X-ray computed tomography apparatus that contains a foil composed of a material that is highly attenuating for X-radiation, that is arranged parallel to the image plane of the X-ray computer tomograph given employment of the phantom, and which has an axial expanse that is small compared to the thinnest slice to be measured and which has an expanse in the direction of the image plane of the X-ray computed tomography apparatus that is on the order of magnitude of a few millimeters. By employing a foil of a material that is highly attenuating for X-radiationxe2x80x94materials whose atomic number amounts to at least 13 are especially suitablexe2x80x94, a significantly higher subject contrast is achieved compared to small balls of aluminum, iron or copper or[, respectively,] compared to an air gap, even though the foil can be very thin. For standard X-ray computer tomography systems, the thickness of the foil is then small compared to the axial expanse of the thinnest slice to be measured when the thickness of the foil amounts to between 10 and 100 xcexcm. Since the expanse of the foil in the direction of the image plane of the X-ray computer tomography apparatus lies on the order of magnitude of a few millimeters, the alignment of the foil parallel to the image plane can be easily accomplished. Second, the small expanse of the foil in the direction of the image plane of the X-ray computed tomography apparatus prevents a lowering of the subject contrast due to the MTF of the X-ray computed tomography apparatus.
Compared to small balls, moreover, foils offer the advantage that they are technologically easy to manufacture and process.
In the interests of an easy manipulation of the phantom, in one version of the invention the foil is accepted in a homogeneous material; homogeneous plastics are particularly suitable, as is a water container.
The inventive phantom thus combines the advantages of small metal balls, namely slight expanse perpendicular to the image plane of the X-ray computed tomography apparatus and high X-ray attenuation, with the advantages of an air gap, namely ease of manufacture and low reduction of the subject contrast due to the MTF of the X-ray computer tomography apparatus. The invention allows all slice thicknesses that are currently standard to be measured with a single phantom since the obtainable subject contrast suffices not only for slice thicknesses in the sub-millimeter range but also for thicker slices.